That would be some challenge - you would probably need to prompt the end user for their ISP's smtp server since most external servers have "relaying denied", as well as their SMTP server user name and password.
A more practical/easier solution would be to send an html request to your hosting server, and have it run a script to send the actual email through your server's smtp provider. That's what our channels that require email do for html/plaintext hybrid emails.
